Surface Area and Volume
Up until now, we've explored the world in two dimensions. Area, shapes, and x-y plots are all good and fine but they're a little flat, aren't they? After all, you can't really play Minecraft in two dimensions. If we crank geometry into third gear, we'll go from kiddy cartoons to sophisticated computer animation, like Paper Mario plopped into a 3D Mushroom Kingdom.
The first step in three-dimensional space is knowing how to look at it. Yes, with your eyes, smarty-pants, but it's more than just that. We may think we see the world in three dimensions, but we really only see it in two. What a bust. That means we need to come up with ways to draw 3D space on a piece of 2D paper.